This invention relates to a novel process for the production of compounds comprising peptide linkages.
It is well known in the art that peptide linkages are typically formed via the coupling of an amine to a carboxylic acid.
Such coupling reactions are often carried out in the presence of so-called xe2x80x9ccoupling reagentsxe2x80x9d, such as oxalyl chloride, EDC (1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride) DCC (dicyclohexylcarbodiimide), isobutyl chlorocarbonate, HBTU ([N,N,Nxe2x80x2,Nxe2x80x2-tetramethyl-O-(benzotriazol-1-yl)uronium hexafluorophosphate]), HATU (O-(azabenzotriazol-1-yl)-N,N,Nxe2x80x2,Nxe2x80x2-tetramethyluronium hexafluorophosphate) or TBTU ([N,N,Nxe2x80x2,Nxe2x80x2-tetramethyl-O-(benzotriazol-1-yl)uronium tetrafluoroborate]), and an appropriate base.
Such agents, which are typically used to enhance the efficiency of the coupling reaction and/or the yield of the xe2x80x9ccoupledxe2x80x9d product, have the disadvantage that they are expensive. Further, they may promote side-reactions and/or the formation of by-products, leading to a coupled product that has to be purified before it can be used, and/or to unacceptable levels of chemical effluent being released into the environment.
Thus, there is a need for an improved, cost-effective process for the formation of peptide linkages. Such a process should preferably minimise the number of reagents that need to be employed, and thus the number of side-reactions and by-products that are formed and need to be removed prior to further processing of the coupled product.
European patent applications EP 168 769 and EP 336 368. and U.S. Pat. No. 5,359,086 disclose the coupling of N-carboxyanhydrides (NCAs) to cyclic amino acids, such as proline and esters thereof. There is no mention in these documents of the coupling of NCAs to cyclic amino acid amides.
We have found, surprisingly, that peptide linkages may be formed by way of an efficient one-step process, in which NCAs are coupled directly to inter alia cyclic amino acid amide derivatives.
According to a first aspect of the invention there is provided a process for the production of a compound of formula I, 
wherein
R1 represents H or C1-3 alkyl optionally substituted and/or terminated with one or more substituents selected from halo, cyano, nitro, S(O)2NH2, C(O)R4, C(O)OR5, SR6, S(O)R7, S(O)2R8, N(R9)R10 or OR11;
R2 represents C1-8 alkyl, xe2x80x94A1-C4-8 cycloalkyl or xe2x80x94A1-C6-10 aryl, all of which are optionally substituted by one or more substituents selected from C1-4 alkyl (which latter group is optionally substituted by one or more halo substituents), methylenedioxy, halo, cyano, nitro, S(O)2NH2, C(O)R4, C(O)OR5, SR6, S(O)R7, S(O)2R8, N(R9)R10 or OR11;
R3 represents N(R12)R13 or CH(R12)R13;
R13 represents, at each occurrence, phenyl, C1-3 alkylphenyl or C(O)phenyl, all of which groups are optionally substituted by one or more substituents selected from cyano, amidino, hydroxyamidino, halo, C1-4 alkyl (which group is optionally substituted by one or more halo group), SR6, N(R9)R10 or OR11;
R9 and R10 independently represent, at each occurrence, H, C1-4 alkyl or C(O)R14;
A1 represents, at each occurrence, a single bond or C1-4 alkylene; n represents 1, 2 or 3;
R5 represents, at each occurrence, H, C1-6 alkyl or C1-3 alkylphenyl, which latter group is optionally substituted by one or more substituents selected from C1-4 alkyl (which latter group is optionally substituted by one or more halo substituents), methylenedioxy, halo, cyano, nitro, S(O)2NH2, C(O)R4, C(O)OR5a, SR6, S(O)R7, S(O)2R8, N(R9)R10 or OR11; R4, R5a, R6, R11, R12 and R14 independently represent, at each occurrence, H or C1-4 alkyl; and
R7 and R8 independently represent, at each occurrence, C1-4 alkyl,
which process comprises reaction of a compound of formula II, 
wherein R1 and R2 are as defined above, with a compound of formula III, 
wherein R3 and n are as defined above,
and which process is referred to hereinafter as xe2x80x9cthe process of the inventionxe2x80x9d.
Alkyl groups that R1, R2, R4, R5, R5a, R6, R7, R8, R9, R10, R11, R12 and R14 may represent, and with which R2, R5 and R13 may be substituted, may be linear or, when there is a sufficient number (i.e. three) of carbon atoms, be branched and/or cyclic. Further, when there is a sufficient number (i.e. four) of carbon atoms, such alkyl groups may also be part cyclic/acyclic. Such alkyl groups may also be saturated or, when there is a sufficient number (i.e. two) of carbon atoms, be unsaturated and/or interrupted by oxygen.
For the avoidance of doubt, the point of attachment of alkylphenyl groups that R5 and R13 may represent is at the alkyl part of such groups. The alkyl part of such groups may be linear or, when there is a sufficient number (i.e. three) of carbon atoms, be branched. Such alkyl parts may also be saturated or, when there is a sufficient number (i.e. two) of carbon atoms, be unsaturated and/or interrupted by oxygen.
Alkylene chains that A1 may represent may be linear or, when there is a sufficient number (i.e. two) of carbon atoms, be branched. Such chains may also be saturated or, when there is a sufficient number (i.e. two) of carbon atoms, be unsaturated and/or interrupted by oxygen.
As used herein, the term xe2x80x9chaloxe2x80x9d includes fluoro, chloro, bromo or iodo.
Preferred values of R1 include C1-3 alkyl groups (e.g. an ethyl or, particularly a methyl group) substituted and/or terminated by a C(O)OR5 substituent, and, particularly, when the alkyl group is a C2-3 alkyl group, those in which the optional C(O)OR5 substituent is not located at the carbon atom to which the NH group is also attached. Preferred values of R5 include H, linear or branched C1-3 alkyl (e.g. iso-propyl or ethyl) or C1-2 alkylphenyl (e.g. benzyl).
Preferred values of R2 include xe2x80x94A1-C5-7 cycloalkyl or xe2x80x94Al-phenyl, in which, in both cases, A1 represents a single bond or C1-2 alkylene. Particularly preferred R2 groups include cyclohexyl.
Preferred values of R3 include N(R12)R13, in which R12 represents C1-2 alkyl or, especially, H, and R13 represents C1-2 alkylphenyl (e.g. benzyl), which latter group is substituted with an amidino group, a hydroxyamidino group or, preferably, a cyano group, which substituent is preferably located at the 4-position on the phenyl part of that group (relative to the C1-2 alkyl part). When R13 represents an alkylphenyl group substituted at the phenyl part with amidino, hydroxyamidino or cyano, further such groups that may be mentioned include those that are also optionally substituted at the phenyl part with one or more halo (e.g. fluoro) atoms.
Preferred values of n include 2 and, especially, 1.
The process of the invention is preferably carried out in the presence of an appropriate base and a suitable solvent system, which solvent system may comprise aqueous and/or organic solvents. The base and solvent system that are employed should not react chemically with, or give rise to stereochemical changes in, the reactants or product once formed, or give rise to other side reactions.
Suitable bases include inorganic bases, such as hydroxides, alkoxides, hydrogen carbonates or carbonates of alkali metals (such as Na or K), or organic bases, such as common tertiary amine bases (e.g. triethylamine, diisopropylethylamine and N-methyl morpholine). Particularly preferred bases include amine bases.
Suitable aqueous solvents include water. Suitable organic solvents include acetates (e.g. ethyl acetate, iso-propyl acetate, butyl acetate), acetonitrile, toluene, dichloromethane, tetrahydrofuran, dimethylformamide and mixtures of any of these solvents.
It is preferred that water is present in the solvent system that is employed. We have found, advantageously, that, when water is employed as a solvent, the efficiency of the process is insensitive to the solid state properties of the amine of formula III that is employed.
NCA compounds of formula II are known in the art or may be prepared using conventional techniques (see, for example, the analogous processes described in German patent application DE 40 11 171 and international patent application WO 96/12729).
For example, compounds of formula II may be prepared by reaction of an amino acid compound of formula IV, 
wherein R1 and R2 are as hereinbefore defined, with phosgene. This reaction may, for example, be carried out at elevated temperature (e.g. between 30xc2x0 C. and reflux temperature) in the presence of a suitable solvent, such as tetrahydrofuran.
Compounds of formula IV, in which R1 represents optionally substituted/terminated C1-3 alkyl and which are optionally in the form of acid (e.g. hydrohalic acid) addition salts, may be prepared by reaction of a corresponding compound of formula IV in which R1 is H, or an acid (e.g. a hydrohalic acid or an alkylsulphonic acid) addition salt thereof, with a compound of formula V,
R1axe2x80x94L1xe2x80x83xe2x80x83V 
wherein L1 represents a suitable leaving group (such as halo (e.g. Cl, Br or I), triflate, tosylate, mesylate, etc.) and R1a represents C1-3 alkyl, optionally substituted and/or terminated with one or more substituents selected from halo, cyano, nitro, S(O)2NH2, C(O)R4, C(O)OR5, SR6, S(O)R7, S(O)2R8, N(R9)R10 or OR11 and R4, R5, R6, R7, R8, R9, R10 and R11 are as hereinbefore defined. This reaction may, for example, be carried out at between room temperature and reflux temperature, in the presence of a suitable solvent system (e.g. aqueous and/or organic solvents (such as tetrahydrofuran, ethyl acetate, toluene, acetonitrile, etc.), including water/tetrahydrofuran mixtures and water/ethyl acetate mixtures), optionally in the presence of a base (e.g. an inorganic base, such as potassium hydrogen carbonate). The skilled person will appreciate that it may be desirable to protect the carboxylic acid group of the respective compound of formula IV with a suitable protecting group (e.g. a benzyl group) before carrying out this reaction.
Compounds of formula IV in which R1 represents H and which are optionally in the form of acid (e.g. hydrohalic acid) addition salts may be prepared by hydrolysis of a corresponding compound of formula II in which R1 represents H under appropriate reaction conditions. The skilled person will appreciate that it may be possible and/or desirable to protect the carboxylic acid group of the resultant compound of formula IV with a suitable protecting group (e.g. a benzyl group) by performing this hydrolysis in the presence of a suitable protecting reagent (e.g. benzyl alcohol).
Alternatively, compounds of formula IV in which R1 represents C1-3 alkyl substituted or terminated by C(O)OR5, provided that, in the case of a C2-3 alkyl group, the C(O)OR5 group is not located at the carbon atom to which the NH group is also attached, may be prepared by hydrogenation of a corresponding compound of formula IV in which R1 represents H in the presence of either a compound of formula VI,
OHCxe2x80x94R1bxe2x80x83xe2x80x83VI 
or a compound of formula VII, 
in which, in both cases, R1b represents C(O)OR5 or C1-2 alkyl substituted or terminated by C(O)OR5 and R5 is as hereinbefore defined, and, in the case of a compound of formula VII, the Ra groups either represent, separately, C 1-4 alkyl groups, or are joined together to form a C2-4 alkylene chain, under appropriate reaction conditions. Reaction of a compound of formula IV in which R1 is H with a compound of formula VI, or a compound of formula VII, may, for example, be carried out in the presence of a suitable solvent system (e.g. an alkyl alcohol (such as methanol, ethanol, i-propanol or n-propanol), which alcohol should be selected to avoid trans-esterification reactions in the reactants or products of the relevant reaction) and, optionally, an appropriate acid, such as an inorganic acid (e.g. HCl) or an organic acid (e.g. methanesulphonic acid, toluenesulphonic acid etc). The skilled person will appreciate that these reactions may be carried out at around room temperature or above (e.g. up to the reflux temperature of the solvent that is employed (e.g. at around 40 to 60xc2x0 C., such as about 50xc2x0 C.)), under hydrogen, for example under a positive pressure of hydrogen (e.g. 3 to 8 (e.g. 4 to 6, such as 5)) atmospheres, and in the presence of an appropriate hydrogenation catalyst system (e.g. Pd/C, Pt/C). The skilled person will appreciate that altering the pressure of hydrogen during the relevant reaction will alter the reflux temperature of the solvent.
According to two further aspects of the invention, there is thus provided a process for the preparation of a compound of formula IV in which R1 represents C1-3 alkyl substituted or terminated by C(O)OR5 provided that, in the case of a C2-3 alkyl group, the C(O)OR5 group is not located at the carbon atom to which the NH group is also attached, which process comprises hydrogenation of a corresponding compound of formula IV in which R1 represents H in the presence of a compound of formula VI as hereinbefore defined, or a compound of formula VII as hereinbefore defined.
These processes for the preparation of compounds of formula IV in which R1 represents C1-3 alkyl substituted or terminated by C(O)OR5 may have the advantage that such compounds may be prepared in fewer steps than other processes for the preparation of equivalent compounds of formula IV, which other processes are either described herein or may be described elsewhere in the prior art. In particular, this process avoids the need for protection of the relevant carboxylic acid group in a compound of formula IV in which R1 represents H prior to coupling with e.g. a compound of formula V, for example as described herein.
Compounds of formula III, V, VI and VII are either commercially available, are known in the literature, or may be obtained either by analogy with the processes described herein, or by conventional synthetic procedures, in accordance with standard techniques, from readily available starting materials using appropriate reagents and reaction conditions.
Substituents on the aryl (e.g. phenyl), and (if appropriate) heterocyclic, group(s) in compounds defined herein may be converted to other substituents using techniques well known to those skilled in the art. For example, amino may be converted to amido, amido may be hydrolysed to amino, hydroxy may be converted to alkoxy, alkoxy may be hydrolysed to hydroxy, and, where appropriate, cyano may be converted to amidino or hydroxyamidino.
It will be appreciated by those skilled in the art that, in the processes described above, the functional groups of intermediate compounds may be, or may need to be, protected by protecting groups. In particular, it may be desirable to protect the carboxylic acid functionality of a compound of formula IV (and acid addition salts thereof), with an appropriate protecting group (e.g. a benzyl group), which should be removed before reaction with phosgene.
Functional groups which it is desirable to protect thus include hydroxy, amino and carboxylic acid. Suitable protecting groups for hydroxy include trialkylsilyl and diarylalkylsilyl groups (e.g. tert-butyldimethylsilyl, tert-butyldiphenylsilyl or trimethylsilyl), tetrahydropyranyl, benzyl and alkylcarbonyl groups (e.g. methyl- and ethylcarbonyl groups). Suitable protecting groups for amino include benzyl, tert-butyloxycarbonyl, 9-fluorenylmethoxycarbonyl or benzyloxycarbonyl. Suitable protecting groups for carboxylic acid include C1-6 alkyl, ally or benzyl esters.
The protection and deportation of functional groups may take place before or after any of the reaction steps described hereinbefore.
Protecting groups may be removed in accordance with techniques which are well known to those skilled in the art and as described hereinafter.
The use of protecting groups is fully described in xe2x80x9cProtective Groups in Organic Chemistryxe2x80x9d, edited by J W F McOmie, Plenum Press (1973), and xe2x80x9cProtective Groups in Organic Synthesisxe2x80x9d, 3rd edition, T W Greene and P G M Wutz, Wiley-Interscience (1999).
Compounds of formula I prepared by way of the process of the invention may be utilised in a subsequent peptide coupling reaction and/or may be converted to other compounds of formula I. For example, compounds of formula I wherein R3 represents N(R12)R13, in which R13 represents phenyl, C(O)phenyl, or, preferably, C1-3 alkylphenyl (e.g. benzyl) substituted (at the respective phenyl parts) by cyano (e.g., in the case of C1-3 alkylphenyl, at the 4-position relative to the C1-3 alkyl part) may be converted to equivalent compounds of formula I in which the relevant phenyl group is substituted by an amidino, or, preferably, a hydroxyamidino, group under techniques known to those skilled in the art (e.g. using hydroxylamine).
The process of the invention is thus useful in the production of chemical compounds comprising peptide linkages and, in particular, peptide compounds comprising cyclic amino acid groups.
The process of the invention possesses the surprising advantage that peptide compounds of formula I may be obtained efficiently from readily-available starting materials without the need for the employment of coupling agents.
Further, the process of the invention may have the advantage that compounds of formula I may be prepared in higher yields, in less time, more conveniently, and at a lower cost, than when prepared in processes described in the prior art.
The invention is illustrated, but in no way limited, by the following examples.